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Have you faced complications from inadequate quenching, tempering, or documentation? You’re not alone. Small oversights can compromise part quality and performance. In this Technical Tuesday installment Ryan Van Dyke, metallurgical engineering manager at Paulo, addresses the top five pitfalls that in-house heat treating operations encounter and when to find another solution.

This informative piece was first released in Heat Treat Today’s July 2025, Heat Treat Super Brands print edition.

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When dealing with high-volume production, running an in-house heat treating operation may seem like it makes financial and logistical sense. The ability to immediately process large batches of the same parts, minimize handling time, and tightly integrate heat treatment into the manufacturing workflow can provide critical advantages over outsourcing.  

Industries involving high-volume machining of parts (e.g., automotive fasteners and bearings) rely on heat treating in-house to maintain efficiency and cost control. When parts are produced in the millions, outsourcing heat treating risks working with an inadequate supplier, introducing unacceptable lead time delays, transportation risks, and logistical complexities that do not align with high-throughput manufacturing.  

Conversely, in-house heat treat operations often lack the flexibility, specialized equipment, and process control systems that commercial heat treaters develop over years of refining best practices. I have worked with countless manufacturers with in-house heat treat who have faced challenges they were unable to solve internally — from unpredictable distortion to process inconsistency, failed audits, and more. When they turn to a commercial heat treater for help, we often find the same core issues at play.  

While commercial heat treating is not always the best fit for high-volume operations, there are real risks if you choose to run heat treating in-house. Here are the five most common pitfalls I’ve seen.  

Pitfall #1: Inconsistent Mechanical Properties 

Understanding the Problem 

Heat treating sets the foundation for a part’s hardness, toughness, and overall performance. This is done by the controlled heating and cooling of materials in a special atmosphere and then locking in the desired microstructure.  

One major challenge that impacts consistency in parts is furnace temperature uniformity. Older or improperly calibrated furnaces can create hot and cold spots, leading to localized variations in hardness and mechanical properties within the same batch. This is a common challenge in-house heat treaters face. To avoid hot spots, heat treaters must go beyond just considering equipment age — they should implement robust preventative maintenance programs and routinely calibrate furnaces to ensure consistent thermal performance across all zones. 

Real-World Consequences 

• Distortion issues from non-uniform heating: Variations in temperature cause inconsistent thermal profiles, leading to unpredictable warping and dimensional instability. For example, a die used for stamping operations requires excessive rework after heat treatment because some areas of the part distorted unevenly due to poor furnace temperature uniformity. 

• Inconsistent hardness in a load: Hot and cold spots in austenitizing and tempering furnaces can cause parts in some areas to have a different final hardness than others. For example, a load of larger diameter structural bolts was tempered in a furnace with poor uniformity. Bolts located in a hot spot in one corner of the furnace showed below specification mid-radius hardness due to over-tempering. 

Pitfall #2: Surface Contamination from Incorrect Gas Atmosphere Control 

Understanding the Problem 

Many manufacturers with in-house heat treating operations use gas atmospheres to control oxidation and facilitate processes like carburizing and nitriding. However, if the gas atmosphere is not properly monitored, it can lead to oxidation, decarburization, or uncontrolled case hardening. 

Heat treaters often rely on Endothermic gas generators that produce a carbon-rich atmosphere. Without precise control of carbon potential, parts may develop non-uniform case depths, excessive soot buildup, or — the opposite extreme — decarburization, in which the surface loses carbon and thus its strength and hardness. Therefore, it’s imperative to monitor and adjust atmosphere parameters in real time using carbon probes to maintain precise control of carbon potential. 

Real-World Consequences 

• Decarburization leading to soft surfaces: If the furnace atmosphere lacks sufficient carbon potential, the steel loses carbon at the surface, reducing hardness and durability. For example, aerospace landing gear components could be rejected if surface hardness tests show excessive decarburization, making them unsuitable for service. 

• Scaling and oxidation issues: Excess oxygen in the furnace leads to surface oxidation, requiring costly post-processing like machining or pickling. For example, stainless steel medical implants can develop scale during heat treatment, requiring extensive rework to restore a clean finish. 

• Uneven carburizing creating case depth variations: Fluctuations in furnace gas composition lead to inconsistent carbon diffusion, making case depth unpredictable. For example, a batch of industrial gears can fail inspection because some parts have insu cient case depth while others are over-cased, leading to production delays. 

Pitfall #3: Suboptimal Quenching Causing Distortion & Residual Stresses 

Understanding the Problem 

Quenching is one of the most stress-inducing steps in heat treatment. Rapid cooling causes phase transformations and volume changes within the steel, leading to internal stresses and distortion.  

Manufacturers with in-house heat treaters often struggle with choosing the right quench medium, optimizing agitation rates, and positioning parts correctly during quenching. Additionally, many only have access to one quench medium, such as oil, and will attempt to apply it to all materials and geometries — even when a slower or faster quench rate is required. This mismatch can cause excessive distortion, high residual stresses, and even quench cracking. 

Another issue is poor part orientation during quenching. If a part is improperly positioned, different areas will cool at different rates, creating non-uniform hardness and residual stress buildup, which can later cause warping or failure in service. 

Real-World Consequences 

• Incorrect quenchant selection: If the wrong quench medium is used, such as oil when polymer or water would be more suitable, the parts could end up having inconsistent hardness in various sections due to insufficient cooling. Conversely, selecting a fast oil as a quenchant when hot oil would be more suitable could cause excessive distortion due to the faster cooling rate. For example, lifting shackles quenched in oil will not have sufficient hardening response throughout the cross-section, causing them to be rejected for service due to low strength values in the center of the part. 

• Insufficient quenchant agitation: If the quenchant in the quench tank is not sufficiently agitated when the parts are submerged, then cooling rates throughout the load of parts could vary, causing different amounts of hardening. For example, parts near the edges of a batch load show hardness testing within specification, while parts in the center of the load show hardness below specification. 

• Incorrect positioning of parts: How a part is oriented during quenching can have a large impact on the amount of distortion after heat treatment. If a part is laid horizontally rather than vertically, the amount of distortion can dramatically increase. For example, if a hollow cylinder was laid horizontally for processing, rather than vertically, the cylinder would likely be at risk of material creep during austenization, as well as deformation from the bottom of the part quenching before the top. The result would be distortion in the inner diameter and along the length in excess of the amount of additional material le for machining, causing the part to become scrap. 

Pitfall #4: Brittle Failures from Inadequate Tempering 

Understanding the Problem 

Tempering is a critical post-quench process that reduces residual stresses and brittleness while fine-tuning hardness and toughness. After quenching, steel is in a highly stressed martensitic state, which, if left untreated, can lead to catastrophic failures in service. 

If heat treaters are working under tight production schedules or have an incomplete understanding of tempering curves for different steels, then they may fall into the trap of rushing or even omitting tempering cycles. For some in-house heat treat operations, a single tempering cycle may be employed when a double temper is required, particularly for high-alloy steels like D2, H13, or certain aerospace-grade alloys. 

Real-World Consequences 

• Brittle fracture under load: If a part is left untempered or under-tempered, the high internal stresses from quenching remain, making it prone to sudden brittle fracture when subjected to impact or fatigue loading. For example, an induction-hardened gear used in heavy machinery can snap under torque loading due to excessive quench-induced stresses. It is very common to skip tempering on induction-hardened parts, especially in in-house heat treat operations where cycle times are minimized as much as possible. 

• Reduced wear resistance due to over-tempering: If a steel is over-tempered (held at too high a temperature or for too long), excessive softening can occur, reducing wear resistance and surface hardness. For example, a die used in stamping operations can wear prematurely because it was tempered above its recommended range, leading to a loss of edge retention. 

• Excessive retained austenite leading to dimensional instability: Some steels, particularly high-carbon and high-alloy grades, require a secondary tempering cycle to stabilize the microstructure. Skipping this can leave excessive retained austenite, which converts to untempered martensite over time, causing unexpected distortion or possibly cracks forming in the material in service. For example, a precision-ground shaft can warp and develop cracks weeks after heat treatment because retained austenite transforms to untempered martensite in service, altering the part’s geometry and encouraging fractures to form. 

Pitfall #5: Lack of Process Documentation & Repeatability Issues 

Understanding the Problem 

Heat treating is a process-sensitive operation where small variations can lead to major differences in final part properties. If a heat treat operation does not have detailed documentation and tracking systems, this will lead to inconsistencies in cycle parameters, atmosphere control, and quenching conditions. 

One of the most common issues is manual adjustments without proper record-keeping, which can lead to process drift. Operators may tweak furnace temperatures, quench delays, or gas flow rates without logging the changes, creating batch-to-batch variability. 

Additionally, compliance and traceability may present a challenge for manufacturers facing ISO, Nadcap, or AS9100 audits. When an auditor asks for process records, lacking verifiable data is a red flag for non-compliance. 

Real-World Consequences 

• Batch-to-batch variability: When process parameters are not documented or followed precisely, parts in one batch may have different hardness, case depth, or dimensional stability than parts in the next batch — leading to field failures or quality escapes. For example, a manufacturer of automotive control arms may and that some components fail impact testing while others pass, leading to a full production hold to investigate process inconsistencies. 

• Failed audits and compliance issues: Without traceable process documentation, heat treat operations can fail compliance audits, especially for industries with strict quality requirements. For example, an aerospace supplier could lose Nadcap certification because they cannot provide accurate records of furnace temperature control, atmosphere composition, and quench parameters for critical landing gear components. 

• Difficulty troubleshooting heat treat issues: When a batch of parts fails post-heat treatment inspection, the root cause can be nearly impossible to determine if there are no detailed process records. For example, a fastener manufacturer might experience high rejection rates due to inconsistent case depths, but if the atmosphere carbon potential wasn’t recorded, they will not be able to pinpoint whether it was a gas mix issue, furnace drift, or soak time variance. 

• Expensive scrap and rework costs: A lack of process repeatability leads to high scrap rates and expensive rework to bring parts back into spec. For example, a tooling manufacturer might have to scrap an entire run of die components after discovering that an unrecorded furnace temperature deviation softened the steel below acceptable hardness levels. 

• Lack of lot traceability: When a heat treatment problem does occur, being able to trace it back to exactly which piece of equipment it ran in and when is critical for determining root cause. For example, many automotive seating brackets exhibit low hardness after heat treatment. However, if lot traceability to the furnace cycle was not maintained, root cause of factors like incorrect furnace temperature, inadequate carbon control, or insufficient quench agitation are much more difficult to identify. 

When To Call a Commercial Heat Treater 

If limited resources and/or lack of specialized expertise are in question, these five pitfalls can easily occur. Even the most well-run in-house heat treat operations must balance production efficiency, heat treat quality, and high-volume demands; additionally, it can be challenging to regularly invest in the most advanced equipment, process monitoring, or specialized personnel. 

There are commercial heat treaters that have built their entire business around controlling these variables with precision. These heat treaters have invested decades into refining their heat treating processes, equipment, and metallurgical expertise to eliminate these issues before they ever become problems.  

If these five pitfalls are ones your operations cannot easily avoid, consider a partnership with the right commercial heat treater to maintain parts with extreme precision, low distortion, and strict compliance specifications.

About The Author:

Ryan Van Dyke is the manager of metallurgical engineering at Paulo, where he works closely with customers to solve challenging thermal processing issues. He’s dedicated to pushing the limits of heat treating performance, continuously innovating more efficient, reliable ways to process critical parts. Ryan was an honoree in Heat Treat Today’s 40 Under 40 Class of 2023. 

For more information: Contact Ryan Van Dyke at RVanDyke@paulo.com. 

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